High-pressure discharge lamps, and particularly high-pressure discharge lamps having a metal halide fill which includes sodium, may, in use of the lamp, be subject to deterioration of the sealing foils due to corrosion. Corrosion attacks may destroy the sealing foils, typically of molybdenum. Attack on the foils causes leakage of the discharge vessel, resulting in failure of the lamp. Corrosion of the sealing foils, it is believed, is caused by thermal diffusion of metal halide compounds from the discharge bulb or vessel into the region of the pinch seal in which the foils are embedded. It has been determined that two adjacently located sealing foils which are at different voltage levels additionally are subjected to corrosion due to yet another effect discussed below. In single-ended lamps, two sealing foils at different voltage levels are located next to each other within a pinch seal.
The book "Electric Discharge Lamps", M.I.T. Press 1971, by John F. Waymouth, states that a direct current voltage may be built between adjacently located foils. The lamp vessel, usually, is quartz glass. Upon heating, quartz glass becomes weakly conductive. Thus, in operation of the lamp, an electrolytic current can flow between the sealing foils. This current causes penetration of metal ions, particularly sodium ions, within the pinch seal, thus causing corrosion of the foils. Both effects are temperature dependent. In lamps of low power, in which the spacing between the sealing foils is small, the second, corrosive, effect is predominant.
German Utility Model 88 05 183 describes a proposed solution to inhibit thermal diffusion of metal halide compounds from the discharge vessel into the region of the pinch seal. The electrode shafts are surrounded in the region of the pinch seal up to the sealing foils by insulating tubes, for example made of quartz glass, which are intended to prevent thermal diffusion of metal halide compounds from the discharge vessel into the region of the pinch seal as much as possible.